1. Field of the Invention
The present invention relates to an obstacle detection apparatus.
2. Description of Related Art
An obstacle detection apparatus for a vehicle has already been put to practical use. Such an obstacle detection apparatus detects a presence of an obstacle around a vehicle and a distance to the obstacle. This type of obstacle detection apparatus is utilized in, for example, a system (e.g., a clearance sonar system) that warns a driver of a vehicle about risk of probable collision between the obstacle and the vehicle. Also, this type of obstacle detection apparatus is utilized in a system (i.e., a parking assist system) that searches for a parking area for parking a vehicle therein, and guides a vehicle to enter the parking area.
Japanese Unexamined Patent Application Publication No. 2006-58281, corresponding to U.S. Pat. No. 7,278,314, has proposed an obstacle detection apparatus that includes a microphone (i.e., transducer) as a wave receiving element. In accordance with characteristics of the microphone, the apparatus is capable of variably setting a transmission wave frequency and a center frequency used in filtering.
In the above obstacle detection apparatus, the transmission wave frequency is variable, and thereby, it is possible to optimize the transmission wave frequency in accordance with characteristics of the microphone. According to the above apparatus, however, since the transmission wave frequency are optimized at a time when the obstacle detection apparatus is activated, the transmission wave frequency is not changed after the activation. In addition, the obstacle detection apparatus is configured such that, when a predetermined condition is satisfied after an obstacle detection operation is started, the transmission wave frequency is not switched.
A sensing range of a conventional obstacle detection apparatus is constant. The sensing range is determined by, for example, a directivity and sensing distance of the ultrasonic sensor.
A characteristic of an ultrasonic sensor required in detecting an obstacle located around the vehicle is typically different from that in detecting an obstacle located distant from the vehicle. Therefore, it has been difficult to improve performance of an obstacle detection apparatus required in both the above cases.
According to an ultrasonic sensor disclosed in Japanese Unexamined Patent Application Publication No. 2006-58281, when emphasis is placed on detecting an obstacle located close to the vehicle, it may be preferable to employ an ultrasonic sensor having a relatively-low directivity (i.e., a wide directivity) because it is possible to reduce an undetectable area around the vehicle. However, when an ultrasonic sensor having a low directivity is used for detecting an obstacle located distant from the vehicle, the ultrasonic sensor may receive a reflected ultrasonic wave coming from an undesired direction. As a result, error detection increases, and it becomes difficult to increase a sensing distance.
Inversely, when emphasis is placed on detecting an obstacle located distant from the vehicle, it may be preferable to employ an ultrasonic sensor having relatively-high directivity (i.e., a narrow directivity) because the ultrasonic sensor can reliably detect an obstacle existing in a desired direction. However, the use of the ultrasonic sensor with a high directivity increases an undetectable area around the vehicle. In other words, according to the ultrasonic sensor, it is difficult to both reduce of an undetectable area around a vehicle and to restrict error detection in connection with a distant obstacle.
Also, when a vehicle approaches an obstacle at a higher speed, the vehicle reaches the obstacle in a shorter time. Accordingly, when a vehicle speed is large, and when an ultrasonic sensor for detection of a nearby obstacle is employed, it is difficult to warn a driver about a probable collision between the vehicle and the obstacle at an appropriate time because the vehicle reaches the obstacle in a short time.
Inversely, when a vehicle speed is low, the vehicle reaches an obstacle in relatively long time. When an ultrasonic sensor for detecting a distant obstacle is employed in the low speed case, a user may be provided with an unnecessary warning notification such as beeping at a too early time.
Incidentally, a parking assist system searches for a parking area with the vehicle moving forward by using an ultrasonic sensor. Then, the parking assist system causes the vehicle to move backwards to enter the parking area. Therefore, it may be possible to use the same ultrasonic sensor as that used in the search for the parking area the When the vehicle moves backward while detecting an obstacle around the vehicle.
However, an ultrasonic sensor for searching for the parking area may typically have characteristics adapted for detecting a distant obstacle. Therefore, although it is possible to employ such an ultrasonic sensor to detect an obstacle located around the vehicle, there exits a large undetectable area around the vehicle.